1. Field of the Disclosure
This invention pertains to a method for preparing a printing form from a photopolymerizable element, and in particular to a method for preparing a relief printing form that includes a layer of a photopolymerizable composition having an elastomeric block copolymer.
2. Description of Related Art
Flexographic printing plates are well known for use in printing surfaces which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive elements containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable elastomeric layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, photopolymerization of the photopolymerizable layer occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. The element is treated with a suitable solution, e.g., solvent or aqueous-based washout, or thermally, to remove the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing.
An alternative to solution development, the thermal development process is desired as the process removes the unexposed areas while avoiding the subsequent time-consuming drying step associated with solution development. In a thermal development process, the photosensitive layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. No. 3,060,023 (Burg et al.); U.S. Pat. No. 3,264,103 (Cohen et al.); U.S. Pat. No. 5,015,556 (Martens); U.S. Pat. No. 5,175,072 (Martens); U.S. Pat. No. 5,215,859 (Martens); and U.S. Pat. No. 5,279,697 (Peterson et al.). The exposed portions of the photosensitive layer remain hard, that is do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated from the photosensitive layer. The cycle of heating and contacting the photosensitive layer with the absorbent material may need to be repeated several times in order to sufficiently remove the flowable composition from the un-irradiated areas and form a relief structure suitable for printing. After such processing, there remains a raised relief structure of irradiated, hardened composition that represents the irradiated image.
However with increasing demands on quality, the current state-of-the-art flexographic printing forms may not perform as desired and have trouble meeting the ever increasing demands on quality. One weakness in many flexographic printing plates, especially in large areas of solid coverage, is poor transfer of ink to the substrate resulting in print defects. This property is usually seen most often in high durometer flexographic printing plates. Unsatisfactory printing results are especially obtained with solvent based printing inks, and with UV-curable printing inks. For flexographic printing plates having poor ink transfer, a higher amount of physical impression is necessary during printing to increase the tonal density of solid image areas. But higher impression leads to higher dot gain in screened image areas, which creates an undesired printed image. Especially when printing on critical printing materials like, for example, foils, a lot of print defects can be observed.
It is desirable for a flexographic printing plate to have a relief structure that can print a complete tonal range that includes dense uniform solid coverage and fine printing elements, such as fine sharp lines and highlight dots, i.e., halftone dots having less than about 5%, and in some cases less than about 2%, of a solid area printed. The conditions associated with thermal development can tend to distort the fine printing elements and highlight dots in the relief structure of the printing plate. In addition, the absorbent material may create marks on the contacted surface of a thermally developed flexographic printing plate, sometimes referred to as web-marks, such that in some cases the marks can be observed in the image printed on the substrate.
So a need arises for a method that is simple and relatively quick in preparing a printing form from a photopolymerizable element. It is desirable for the method to prepare the printing form to have a relief structure that improves transfer of ink to the substrate. It is desirable for the method to prepare a printing form having a relief structure capable of printing a full tonal range including printing of fine print elements and highlight dots and thereby providing improved print quality. It is also desirable for the method to prepare a printing form that is not readily susceptible to marking from the absorbent material associated thermal development, or, if markings from the absorbent material are present on the printing form, the markings are not observed in the image printed by the printing form.